1. Field of the Invention
The present invention relates to the conversion of carbonaceous materials into gaseous or liquid products which may be used as fuels or as chemical intermediates. The starting carbonaceous materials include coal, lignite, char, municipal and agricultural waste, sewage sludge, shale oil and heavy petroleum fractions. The well known processes for converting these materials into gaseous products involve causing the carbonaceous material to react at elevated temperatures with mixtures of steam and air or steam and oxygen or merely air in order to produce a synthesis gas containing carbon monoxide and hydrogen, and often also containing methane, other hydrocarbons and tar. The principal chemical reactions involved in such gasification are discussed below under Prior Art.
Prior Art
Previous disclosures are legion for methods of gasifying carbonaceous materials such as coal. Generally such methods teach heating the carbonaceous materials in gases such as steam (H.sub.2 O) or hydrogen, often at elevated temperatures and pressures; high temperatures and pressures being desired to accelerate rate of chemical reaction between gas and the carbonaceous material. In the following disclosure the carbon in carbonaceous materials will be of major concern and, in writing chemical reactions or discussing mass balance, such carbon will be represented by the chemical symbol, C. The gasification of such carbonaceous materials with steam may be represented by the chemical reactions: EQU (I) C+H.sub.2 O.fwdarw.CO+H.sub.2 .DELTA.F.degree.=+21.8 kcal .DELTA.H.degree.=+31.4 kcal/mole EQU (II) C+2H.sub.2 O.fwdarw.CO.sub.2 +2H.sub.2 .DELTA.F.degree.=+14.9 kcal .DELTA.H.degree.=21.5 kcal/mole
Here .DELTA.F.degree. and .DELTA.H.degree. represent respectively the free energy and enthalpy changes of the chemical reactions under standard conditions of 1 atmosphere pressure and 25.degree. C. Because .DELTA.F.degree. is positive at room temperature it is necessary to conduct the above reactions at elevated temperatures (above 900.degree. C.) to realize practical equilibrium yields; .DELTA.H remains positive, even at higher temperatures, and this enthalpy of reaction is very often supplied by combusting a portion of the carbonaceous material according to the equation: EQU (III) C+O.sub.2 .fwdarw.CO.sub.2 .DELTA.F.degree.=-94.3 kcal .DELTA.H.degree.=-94.1 kcal
The invention which I disclose below permits gasification at low temperature by overcoming the positive .DELTA.F.degree. of reactions I and II by an applied electromotive force during electrolysis in an aqueous electrolyte. My method of electrochemical gasification may also supply the enthalpy of gasification at least in part by electrical means. Most important, this new method of gasification which I describe below permits the production of H.sub.2 as a relatively pure gas whereas the prior-art methods of gasification ordinarily produce a so called "synthesis gas" mixture comprising H.sub.2, CO.sub.2, CO and other components.
Other relevant prior art is that of Vaaler [J. Electrochem Soc. 107, 691-698 (1960); Electrochem Technology 5, 170-173 (1967)] and Janssen and Hoagland [Electrochim. Acta 14, 1097-1108 (1969)], who observed and disclosed that carbonaceous electrodes are consumed during brine electrolysis to produce CO.sub.2. Other relevant work is that of Binder et al [Electrochimica Acta 9, 255-274 (1964)] who observed that in aqueous solutions of sulfuric and phosphoric acid various carbonaceous materials such as active carbon, soot, charcoal and graphite could be anodically oxidized in aqueous electrolytes to CO.sub.2 while working at potentials below that which causes the evolution of oxygen. Working with aqueous electrolytes of sulfuric and phosphoric acid Binder was able to convert up to 80% of his carbon to CO.sub.2 working at temperatures of 55.degree. to CO.sub.2 working at temperatures of 55.degree. to 100.degree. C. Whereas the investigators mentioned just above observed and recognized that carbonaceous materials can be oxidatively consumed when anodically polarized during electrolysis, they all failed to perceive that the oxidative consumption of the carbonaceous matter at the anode might permit the liberation of hydrogen at the cathode with a far lower consumption of electrical energy than in the case where water is electrolyzed using non-consumable electrodes. My invention takes advantage of the anodic consumption of carbonaceous materials during electrolysis of an aqueous electrolyte to produce an amount of hydrogen at the cathode that, in terms of its available chemical free energy, exceeds the quantity of electric energy thereby consumed in the process. The source of the available free energy thereby produced in the form of hydrogen is in part the anodic oxidation of the carbonaceous material and in part the electrical energy supplied to the electrolysis process.
In the subsequent discussion the standard electrochemical potential of a reaction is symbolized by E.degree. and refers to one atmosphere pressure and 25.degree. C. The relationship between .DELTA.F.degree. and E.degree. is .DELTA.F.degree.=-nFE.degree., where n is the number of electrons involved in the chemical reaction and F is the Faraday constant and equal to about 96,500 coulombs per equivalent.